It is known to produce chlorine dioxide by reduction of an acid aqueous sodium chlorate solution using methanol, as described in U.S. Pat. No. 2,881,052. The process, however, is quite slow, involves the handling of a large volume of liquid effluent and the efficiency of the process is quite low. More recently there issued U.S. Pat. No. 4,081,520, assigned to the assignee hereof, wherein the problems of the prior process were overcome by the use of a single vessel generator-evaporator-crystallizer. The latter process operates at high efficiency, produces no liquid effluent and has an acceptable production rate.
In the commercial implementation of the above-noted process, complete loss of chlorine dioxide production has been observed from time-to-time. This phenomenon is thought to arise from complete exhaustion of the trace quantities of chloride ions, which must be present for reaction to occur, as a result of some change in the process conditions of the reaction medium.
It is well known that all chlorine dioxide generating processes involving reaction with chlorate proceed in accordance with the reaction: EQU ClO.sub.3.sup.- +Cl.sup.- +2H.sup.+ .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O
In the processes of U.S. Pat. Nos. 2,881,052 and 4,081,520, the chloride ions are formed in situ by reaction of methanol with the co-produced chlorine, so that significant quantities of chlorine do not result as a by-product and hence the chlorine dioxide contains little or no chlorine, depending on the overall efficiency of the process. If, for some reason, therefore, all the chloride ions are consumed by the above reaction, then production of chlorine dioxide will cease until chloride is produced by reduction of chlorate by methanol. This loss of chlorine dioxide production is termed a "white-out".